Recently a drastic progress is made in the field of the digital radio communication system. In the digital radio communication system, it is necessary that the timing phase be estimated at high speed and high precision at the reception side. Such timing phase estimation is disclosed, for example, in Japanese Laid-open Patent No. 6-141048. This reference discloses a signal detecting system and a burst demodulating apparatus. A correlation between a signal obtained by nonlinear processing, such as enveloping determined from the received base band signal, and a symbol frequency component generated from an asynchronous sample clock is obtained, and timing phase of the asynchronous sample clock and received base band signal are estimated.
The block diagram in FIG. 10 shows a part of a conventional digital radio communication system. This conventional digital radio communication system has a conventional demodulator that in turn has a conventional timing reproducing unit. An antenna 101 receives a burst signal in a PSK modulated RF (radio frequency) band. A frequency converter 102 converts the received RF band signal into a base band signal (received base band signal Sb).
An A/D converter 103a, using an asynchronous sampling clock CK having a frequency of n (n=2) times of symbol rate issued from an oscillator 106, samples the received base band signal Sb at time t (=τ+iT/2). The A/D converter 103a outputs an in-phase component of the sampled received base band signal Sb as in-phase data series Ii. Here, “i” is a natural number, “T” is a symbol period, and “τ” is a timing error (−T/2≦τ<T/2). Similarly, an A/D converter 103b, using an asynchronous sampling clock CK having a frequency of two times of symbol rate issued from the oscillator 106, samples the received base band signal Sb at time t (=τ+iT/2). The A/D converter 103b outputs an orthogonal component of the sampled received base band signal Sb as orthogonal data series Qi.
A timing reproducing unit 104 calculates a timing error τ from the in-phase data series Ii and orthogonal data series Qi. On the other hand, a data interpolating unit 112 is used when the sampling speed is low, and the reception data composed of in-phase data series Ii and orthogonal data series Qi sampled by the asynchronous sampling clock CK with time resolution of n [sample/symbol] is issued by interpolating with the reception data having a time resolution n′ (n′>n) [sample/symbol] each. The data interpolating unit 112 does not interpolate when the sampling speed is high.
A data judging unit 113 extracts the Nyquist point of the reception data interpolated by the data interpolating unit 112 by using the timing error X entered from the timing reproducing unit 104, and outputs the judged value of the reception data at this extracted Nyquist point as demodulated data D1.
On the other hand, an envelope detector 105 of the timing reproducing unit 104 outputs an envelope data Ei showing an envelope of the received base band signal Sb, from the in-phase data series Ii and orthogonal data series Qi, according to equation (1).Ei=((Ii)2+(Qi)2)1/2  (1)
The oscillator 106 outputs an asynchronous sampling clock CK having a frequency of n (n=2) times of symbol rate. A complex sine wave generator 107 operates an m-bit counter by using the asynchronous sampling clock CK, and outputs cosine signal data Ci of symbol frequency fs (=1/T) and sine signal data Si of symbol frequency fs, according to the m-bit counter value Yi ranging from 0 to 2π. Herein, the relation between n and m is n=2m, and Yi . . . Yi∈{0, 1, 2, 3, . . . , 2m−1}. That is, the complex sine generator 107 generates symbol frequency component exp [j2π·fst] of symbol frequency fs, and sine signal data Si and cosine signal data Ci are respectively issued as equation (2) and equation (3).Ci=cos (Yi/2m−1)π  (2)Si=sin (Yi/2m−1)π  (3)
The cosine signal data Ci n-times over-sampled data series of cosine component cos (2π·fst) of symbol frequency fs, and the sine signal data Si is n-times over-sampled data series of sine component sin (2π·fst) of symbol frequency fs.
A correlation value calculator 108 multiplies the envelope data Ei issued from the envelope detector 105, and cosine signal data Ci and sine signal data Si issued from the complex sine wave generator 107 according to equation (4) and equation (5), and determines multiplication results MCi, MSi.MCi=Ei×Ci  (4)MSi=Ei×Si  (5)
Further, the correlation value calculator 108 averages the multiplication results MCi, MSi individually by L symbol time, and outputs correlation signals CIi, SIi according to equation (6) and equation (7).CIi=(MCi+MCi−1+. . . +MCi−nL+1)/L  (6)SIi=(MSi+MSi−1+. . . +MSi−nL+1)/L  (7)
An inverse tangent calculator 109 determines the timing phase difference Δθ of the correlation signals CIi, SIi, according to equation (8).Δθ32 tan−1(SIi/CIi)  (8)
The inverse tangent calculator 109 outputs the timing error τ based on this timing phase error Δθ.
The timing reproducing unit 104 process as explained above when the reception timing of burst signal is known, but when the reception timing of burst signal is unknown and it is required to establish the burst timing synchronism of the burst signal, the burst signal is detected, and the burst timing can be established based on the detected information. In this case, a vector length calculator 110 in the timing reproducing unit 104 determines and outputs the vector length Vi indicated by the correlation signals CIi, SIi issued by the correlation value calculator 108, according to equation (9).Vi=(CIi2+SIi2)1/2  (9)
A comparator 111 compares the vector length Vi and threshold ε, and when the vector length Vi is larger than the threshold ε, it is judged that the burst signal has been received, and signal detection information D2 of logic “1” is issued, and when the vector length Vi is smaller than the threshold ε, it is judged that no signal has been received, and signal detection information D2 of logic “0” is issued.
Thus, the timing reproducing unit 104 samples asynchronously at speed of n times of the symbol rate, and calculates the correlation by the symbol frequency component by using the sampled information, so that the timing phase can be estimated at high precision and high speed.
As mentioned above, the asynchronous sampling speed is n times of the symbol rate, but since the lower limit value of n is 4 in order to realize the correlation calculation by the correlation value calculator 108 according to equations (4) to (7), the timing phase difference Δθ cannot be calculated by using a smaller value for n.
For example, at n =4, the cosine signal Data Ci to be used is {1, 0, −1, 0, . . . }, and the sine signal data Si is {0, −1, 0, −1, . . . }, and thus the multiplication operation is easy at n=4, but at n=1, the cosine signal data Ci is {1, −1, 1, −1, . . . } and the sine signal data Si {0, 0, 0, 0, . . . }, and although the correlation signal CIi at the in-phase component side is determined, but the correlation signal SIi at the orthogonal component side is always 0, not expressing the true value, and there are two values for the timing phase difference Δθ, that is, {0, π}. In this case, the timing error of maximum ±T/2 occurs regardless of the state of C/N (ratio of carrier signal to noise signal).
Recently, on the other hand, a keen attention is given toward sa wide-band radio communication system which realizes transmission of moving images or transmission of huge amount of data transmission by means of radio circuit. In this wide-band radio communication system, the data transmission speed must be increased from the conventional speed range of tens to hundreds of kbps mainly used for audio communication, to a speed range of tens to hundreds of Mbps.
However, depending on the devices using CMOS gate array or the like, the maximum operating speed of the demodulating apparatus is tens to hundreds of Mbps, and therefore when the data sampling speed (speed of n times of symbol rate) becomes higher, the sampling speed may exceed the maximum operating speed of the demodulating apparatus, and the demodulating apparatus for performing such timing reproduction operation cannot be applied in the wide-band radio communication system.
For example, in the case of QPSK modulation system with data transmission speed of 100 Mbps, that is, the symbol rate of 50M bands, when the demodulating apparatus composed of digital circuits is realized by using the device with the maximum operating speed of 150 MHz, since n≧4, the lower limit value of the sampling speed is 50×4=200 MHz, and this sampling speed exceeds the maximum operating speed of the device of 150 MHz, and the demodulating apparatus performing such timing reproduction operation cannot be applied in the wide-band radio communication system.